Optical fibers commonly comprises a glass core (typically with a diameter of about 120-130 μm), inside which the transmitted optical signal is confined, surrounded by a cladding, preferably made of glass. The combination of core and cladding is usually identified as “optical waveguide”, and is usually produced by chemical reactions according to known processes, such as those known as VAD, OVD, PCVD or MCVD. The optical waveguide is generally protected by an outer coating, typically of polymeric material. This protective coating can comprise a first coating layer positioned directly onto the glass surface, also known as the “primary coating”, and a second coating layer, also known as “secondary coating”, disposed to surround said first coating layer.
These polymer coatings may be obtained from compositions comprising oligomers and monomers that are generally crosslinked by means of UV irradiation in the presence of a suitable photo-initiator. The two coatings described above differ, inter alia, in the mechanical properties of the respective materials.
The material which forms the primary coating is a relatively soft material, with a relatively low modulus of elasticity at room temperature (typically of from 1 MPa to 2 MPa), in order to cushion the glass core and to avoid the microbending phenomena, which attenuate the optical signal and reduce the signal transmission capability of the glass fiber.
The material which forms the secondary coating is relatively harder, having higher modulus of elasticity values at room temperature (typically of from 500 MPa to 2000 MPa), to confer a good mechanical resistance of the optical fiber to the external stress during the installation and working conditions.
U.S. Pat. No. 5,214,734 describes optical fibers which include polymeric jackets for environmental protection and protection against handling. By adding particles of an appropriate material to the polymeric jacket material of an optical fiber, it is possible, to at least some extent, to protect the fiber from loss of strength which is related to exposure of the fiber to moisture. At least one primary and/or secondary layer is filled with particulate, e.g. fumed, silica. The silica-filled layer may be the sole layer of the jacket, as is currently preferred, or, alternatively, it may be any layer or layers of a multiple-layer jacket. About 0.5%-1% silica by weight is sufficient to bring about an increase in the onset of accelerated fatigue when added to the exemplary prepolymer material.
U.S. Pat. No. 5,558,937 discloses a curable thiol-ene composition, specially adapted for use as a primary coating on optical fibers, which comprises a polythiol and a compound having a plurality of norbornene groups thereon, characterized in that one of either the compound having the plurality of norbornene groups or the polythiol has a backbone of a poly(tetramethylene oxide), or is an oligomer thereof, and the poly(tetramethylene oxide) has a molecular weight of between 250 and 5,000. The formulations can be cured using low intensity UV lamps. The cured products are reported to have excellent low temperature flexibility, good humidity and water absorption resistance and good thermal oxidative stability.
No indication is provided about the behavior of the optical fiber in a hot, damp environment. No indication is provided about the composition of the secondary (outer coating).
U.S. Pat. No. 4,525,026 discloses an optical fiber having a single or double coating, wherein the light transmitting fiber is surrounded with one or more protective layers which contain finely divided particles. The particles are typically of a metal or metal oxide matching the metal or metal oxide constituent of the outer portions of the optical fiber itself and are typically suspended in the polymeric, buffering layer directly surrounding the optical fiber. In a particular embodiment a protective material (metallic aluminum, tin oxide and titanium oxide) is put between the cladding and the buffer material in order to saturate or neutralize the environmental fluids before they reach the fiber.
The presence of a protective material layer implies a further manufacturing step and additional costs also due to the necessity of having an oxide with a high degree of purity.